Immune mechanisms by which topically applied vitamin D acts as a therapeutic agent

Topical creams containing vitamin D derivatives or analogues (calcitriol, calcipotriene) are currently used to treat different skin conditions including psoriasis and vitiligo with some success. The skin diseases that can be treated by topical vitamin D are characterised by irregular proliferation and differentiation of keratinocytes accompanied by inflammation and an influx of immune effector cells. We are now starting to better understand the mechanisms by which vitamin D applied to the skin affects keratinocyte growth and reduces the inflammation and immune responses, which drive these and other diseases. Here we outline some of the processes by which vitamin D modulates inflammatory processes that occur in diseased skin, and immune mechanisms that may be activated by vitamin D in the skin and draining lymphatic tissue to control inappropriate inflammatory responses. In particular, we focus on the cell-driven immune responses mediated by keratinocytes, dendritic cells and T cells that may be targets of vitamin D.